1. Field of the Invention
The present invention relates to a semiconductor laser device having an index-guided structure.
2. Description of the Related Art
S. Nakamura et al. (xe2x80x9cInGaN/GaN/AlGaN-Based Laser Diodes Grown on GaN Substrates with a Fundamental Transverse Mode,xe2x80x9d Japanese Journal of Applied Physics, vol. 37 (1998) L1020-L1022) disclose a short-wavelength semiconductor laser device which emits laser light in the 410 nm band.
This semiconductor laser device is formed as follows. First, a GaN substrate is formed by forming a first GaN layer on a sapphire substrate, selectively growing a second GaN layer by using a SiO2 mask, and removing an excessive portion of the second GaN layer above the top surface of the SiO2 mask. Then, an n-type GaN buffer layer, an n-type InGaN crack preventing layer, an n-type AlGaN/n-type GaN modulation-doped superlattice cladding layer, an n-type GaN optical waveguide layer, an n-type InGaN/InGaN multiple quantum well active layer, a p-type AlGaN carrier block layer, a p-type GaN optical waveguide layer, a p-type AlGaN/GaN modulation-doped superlattice cladding layer, and a p-type GaN contact layer are formed on the above GaN substrate. In addition, an index-guided structure is realized by forming a ridge structure having a width of about 2 micrometers. However, since it is very difficult to control the etching depth, the maximum output power in the fundamental transverse mode is at most about 30 mW. In the above semiconductor laser device, the contact area between the p electrode and the p-type GaN contact layer is small, and therefore the contact resistance and heat generation are great. Therefore, it is difficult to increase the output power.
In addition, as disclosed in Japanese Unexamined Patent Publication, No. 9 (1997)-307190, in the conventional GaN-based index-guided semiconductor laser devices, the index-guided structure is realized by the difference in the refractive index between an AlGaN current confinement layer and a cladding layer. However, when a difference between equivalent refractive indexes is increased to a large value in order to obtain a high quality laser beam by current confinement using the AlGaN current confinement layer, the relative composition of aluminum in the AlGaN current confinement layer becomes greater than that in the cladding layer. Therefore, it is difficult to form the AlGaN current confinement layer with a sufficient thickness.
In order to solve the above problem, Japanese Unexamined Patent Publication, No. 11(1999)-204882 discloses a semiconductor laser device having a ridge-type index-guided structure realized by an AlGaN current confinement layer, and the current confinement layer is realized by a thick superlattice structure. In this semiconductor laser device, an attempt to decrease the contact resistance between the electrode and the contact layer is made in order to avoid the aforementioned problem of the heat generation due to the contact resistance. However, since the index-guided structure is realized by forming the ridge, the contact area is small, and therefore the contact resistance cannot be sufficiently decreased. In addition, since the stripe area should be formed corresponding to an undefective region of the GaN layer, and the undefective region has a width of about 2 micrometers, the maximum possible width of the stripe area is about 2 micrometers. Therefore, it is difficult to realize a wide-stripe high-power semiconductor laser device.
The object of the present invention is to provide a semiconductor laser device which can oscillate in a fundamental transverse mode even when output power is high, and output a high-quality Gaussian laser beam.
According to the present invention, there is provided a semiconductor laser device comprising a GaN layer of a first conductive type; an active layer; a first upper cladding layer of a second conductive type; a current confinement layer of the first conductive type; a second upper cladding layer of the second conductive type; and a GaN contact layer of the second conductive type. In the semiconductor laser device, the active layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the GaN contact layer are formed above the GaN layer; a groove is formed through the full thickness of the current confinement layer so as to form an index-guided structure; the active layer is a single or multiple quantum well active layer formed by alternately forming at least one Inx1Ga1xe2x88x92x1N well and a plurality of Inx2Ga1xe2x88x92x2N barriers, where 0xe2x89xa6x2 less than x1 less than 0.5; the current confinement layer has a superlattice structure formed with Ga1xe2x88x92z4Alz4N barriers and GaN wells, where 0 less than z4 less than 1; the second upper cladding layer is formed over the current confinement layer so as to cover the groove; and the GaN contact layer is formed on the entire upper surface of the second upper cladding layer. In the active layer, the Inx2Ga1xe2x88x92x2N barriers are arranged in both of the outermost layers of the single or multiple quantum well active layer.
Due to the above construction, the semiconductor laser device according to the present invention can oscillate in a fundamental transverse mode, and output a high-quality Gaussian laser beam even when output power is high.
In particular, since the active layer is a single or multiple quantum well active layer formed by alternately forming at least one Inx1Ga1xe2x88x92x1N well and a plurality of Inx2Ga1xe2x88x92x2N barriers, the probability of occurrence of a crystal defect can be reduced, and the semiconductor laser device according to the present invention can generate a reliable short-wavelength laser beam. Further, when the active layer is a multiple quantum well active layer, the characteristics of the semiconductor laser device can be improved. For example, the threshold current can be reduced.
When an aluminum-rich GaAlN material is used in a layered structure made of GaN-based materials, the lattice mismatch occurs, and it is difficult to obtain a highly reliable, high-quality semiconductor laser device. However, since, according to the present invention, the current confinement layer has a superlattice structure formed with Ga1xe2x88x92z4Alz4N barriers and GaN wells, and 0 less than z4 less than 1, it is possible to form the current confinement layer with a thickness equal to or greater than a critical thickness, i.e., the thickness of the current confinement layer can be sufficiently increased so that a desired difference in the equivalent refractive index can be achieved.
Further, since the second upper cladding layer is formed over the current confinement layer so as to cover the groove, and the contact layer is formed on the entire upper surface of the second upper cladding layer, the contact area between the second upper cladding layer and the contact layer can be increased, and the contact resistance can be reduced. Therefore, the emission efficiency can be increased, and the threshold current can be reduced. In particular, when the output power is high, it is possible to reduce heat generation in and near the electrode. Thus, it is possible to prevent deterioration of the semiconductor layers and the electrode due to the heat generation.
Since the index-guided structure is realized by the internal confinement structure, the width of the groove can be adjusted by etching with high accuracy, and therefore a desired stripe width can be realized. Thus, the semiconductor laser device according to the present invention can generate a high-quality laser beam.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) to (iv).
(i) The Ga1xe2x88x92z4Alz4N barriers in the current confinement layer may be doped with a dopant of the first conductive type.
(ii) The Ga1xe2x88x92z4Alz4N barriers and the GaN wells in the current confinement layer may be doped with a dopant of the first conductive type.
In either of the cases (i) and (ii) , a desired difference between the equivalent refractive index of the portion of the active layer under the groove and the equivalent refractive index of the other portion of the active layer under the current confinement layer other than the groove can be obtained, and therefore a high-quality laser beam can be obtained.
(iii) The groove may have a width equal to or greater than 1 micrometer, and smaller than 3 micrometers, and the difference between the equivalent refractive index of the portion of the active layer under the groove for light in a propagation mode in the thickness direction and the equivalent refractive index of the other portion of the active layer under the current confinement layer (other than the groove) for the light in the propagation mode in the thickness direction may be in a range of 0.001 to 0.007.
In this case, the semiconductor laser device according to the present invention can oscillate in the fundamental transverse mode which is controlled with high accuracy.
(iv) The groove may have a width equal to or greater than 3 micrometers, and the difference between the equivalent refractive index of the portion of the active layer under the groove for light in a propagation mode in the thickness direction and the equivalent refractive index of the other portion of the active layer under the current confinement layer (other than the groove) for the light in the propagation mode in the thickness direction may be in a range of 0.001 to 0.02.
In this case, it is possible to avoid the instability of the transverse modes due to the higher-mode oscillation.
The first conductive type is different in carrier polarity from the second conductive type. That is, when the first conductive type is n type, the second conductive type is p type.